The Callisto Rocket Engine is a highly reusable rocket engine that is closed cycle and uses Liquid Hydrogen as a fuel, and Liquid Oxygen as an oxidizer. It uses turbines to spin its turbopumps to accelerate the fuel and oxidizer through the pipes at very high speeds, to supply a significant amount of fuel to the fuel injector, to then be sprayed into the combustion chamber. The fuel and oxidizer sprayed out of the fuel/oxidizer injector are ignited via a chemical ignition (for now). The chemical used is Triethylborane because it is hypergolic with liquid oxygen, which is the oxidizer used. This means, upon the Triethylborane (Teateb) contacting the Liquid Oxygen, they ignite, also igniting the fuel, enabling the engine to support stable combustion. As you can tell by the exhaust gases getting squished inwards upon leaving the nozzle extension, the nozzle is overexpanded. This means, the exhaust gases at the end of the nozzle, being accelerated and leaving the nozzle, are at a lower pressure than Earth’s atmospheric pressure. This causes Earth’s atmospheric pressure to crush the exhaust gases inwards. If the exhaust at the end of the nozzle, leaving the nozzle, is below 40% of Earth’s atmospheric pressure, your nozzle is too overexpanded, which is why we make it just above 40%. If your nozzle is too overexpanded, you get what’s called flow separation, when the atmosphere starts to creep up inside the nozzle walls, creating an instability in the nozzle as it shakes up the nozzle while the exhaust gases move around, leading to an explosion or major failure. Preburner exhaust gases are ran along the walls of the chamber and nozzle to cool it with the cooler preburner exhaust gas. The oxidizer and fuel tanks are pressurized autogenously, meaning the Liquid Hydrogen tank pressurizes itself by letting some Hydrogen boil off to expanding gaseous hydrogen, leading to it pressurizing the tank, and the same goes with the Oxygen Tank. Instead of venting out gaseous oxygen or gaseous hydrogen to control pressure or to relieve pressure, they are put into a separate tank to repress the tanks later instead of letting more hydrogen or oxygen boil off.